Halo Reach is, by far, my favorite game in the entire franchise. You play Noble 6, a mystery-type of sorts who joins Noble Team. Throughout this game, you play a losing battle, doing your best to fight for humanity's survival.
The final level of this game is probably one of my favorite missions of all time in video games. Your team is dead (mostly), humanity just barely manages to escape the Covenant's grasp...and then there's you, the only remaining Spartan, and possibly human, on the planet Reach, fighting an entire army on your own. There is no winning objective, no checkpoints, you just try to fight and survive for as long as possible until your inevitable death. It's dark and sad, and that's exactly what war is in the end.
And that's not to mean that the rest of the game isn't good, the rest of the game is also great! You meet great characters, however the game is still showing the darker side of war, and most of them die in combat (and not all of them die a "Heroes Death").
To get away from the sad bits, the game does feature a character customization area, where you get to customize your Spartan's armor, color and emblem. I think this is suuuuper fun, especially since in the storymode you get to see your Spartan with the armor, colors and emblem that you chose. Not just in game, but in actual cutscenes as well.
Some have complained that there isn't much depth in the customization, but I'm happy we got to do this at all. However, I will note that the "Credits" are a tad bit annoying. You see, you play missions, and depending on how well you did or how many challenges you completed, etc...you earn credits. Credits earn you higher ranks so you can unlock more armor of which you spend your credits on. However, it's...oddly difficult to get credits. It becomes such a task, and you'll end up playing a mission hundreds of times over just to get a decent number of credits, of which you'll spend on an armor piece just to do it all over again.
Although, that's a small complaint to make in this game. This game is beautiful, fun and...just awesome overall! Even if you haven't played much Halo before, I have to say...BUY THIS GAME!!!